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Topic: which element that has more powerful flame temperature.. .potassium or magnesium (Read 2891 times)

If we say ....k flame temperature has 771c and Mg flame temperature has 3100cBut in series reactive table in air reaction.......potassium will be the first element on the top of the chart for the most burning element ...can you explain.Here is an example

As we have noted in previous threads (http://www.thenakedscientists.com/forum/index.php?topic=49202.msg435285#msg435285), activity (how easily something reacts) is not the same as how much heat is released. The heat of formation of MgO is much greater than for K2O (–601 kJ/mol vs –361 kJ/mol) when we adjust for heat released per mass of metal burned, the difference is even more dramatic (–25 kJ/g for Mg vs –4.6 kJ/g for K).

As we have noted in previous strings (http://www.thenakedscientists.com/forum/index.php?topic=49202.msg435285#msg435285), activity (how easily something reacts) is not the same as how much heat is released. The heat of formation of MgO is much greater than for K2O (–601 kJ/mol vs –361 kJ/mol) when we adjust for heat released per mass of metal burned, the difference is even more dramatic (–25 kJ/g for Mg vs –4.6 kJ/g for K).

There is even other that you have to consider: MgO immediately formed in the combustion doesn't vaporize, K2O does and this absorbs heat subtracting it to the flame, I suppose.

I would expect the vaporization of the metal to increase the rate of reaction, thereby increasing the rate at which heat is released, and increasing the apparent temperature of the flame. Though the heat of vaporization of potassium is about 89 kJ/mol (and two equivalents required to compare to heat of formation of K2O2, which is –465 kJ/mol), the product of reaction is still solid K2O2 (I don't think K2O2 has a significant vapor pressure, even at 1000 °C), so even though there is a higher energy intermediate (K(g)), the excess energy is released upon turning into K2O2(s).

Note: I initially assumed that K2O would be the product of reaction, but after looking into it further, it appears that K2O2 is the major product...

I still think the main reason the Mg flame is hotter is because of the difference in heat of reaction per gram of material.

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